The present invention relates to an optical surge suppressing apparatus for suppressing an optical surge generated upon steep level variations in optical input signal.
In recent years, introduction of a long-distance transmission system using optical amplifiers instead of some regenerative repeaters is examined along with the development of optical amplifiers such as an erbium-doped optical fiber amplifier (EDFA).
Since an optical amplifier actually used in the transmission system must have stable amplification characteristics, it is undergone gain control such as (I) constant optical output control and (II) constant gain control. It is generally known in the constant optical output control and constant gain control that an optical surge is often generated upon an optical input hit or steep level variations resulting from disconnection of a transmission path, an apparatus fault, erroneous insertion/removal of a connector, or the like.
FIGS. 5A to 5C show an example of operation characteristics in an optical amplifier using constant optical output control.
When an input hit for a time Ts (sec) shown in FIG. 5A occurs, the optical amplifier must increase the gain in order to keep an output constant regardless of the absence of any input signal, as shown in FIG. 5B. However, when an input signal returns to the original level at time t1 after the hit time, an optical surge is generated as shown in FIG. 5C. The optical surge poses many problems including burnout of the end face of an optical connector and destruction of a light-receiving unit.
As a method of suppressing such an optical surge, Japanese Patent Laid-Open Nos. 7-240717 and 8-18136 disclose the following techniques.
Japanese Patent Laid-Open No. 7-240717 discloses a method of suppressing an optical surge by switching between two control methods. That is, an input level is detected and compared with a predetermined reference value.
(I) If the input level is higher than the reference value, an optical output is controlled constant.
(II) If the input level is lower than the reference value, control is switched to constant gain control (limiter operation or stop of the amplifier).
Japanese Patent Laid-Open No. 8-18136 discloses a method of suppressing an optical surge by monitoring the output light level and gain of an optical fiber amplifier and comparing them with corresponding setting errors to select constant optical output control operation or optical gain limiter operation. An optical surge waveform indicated by the chain line in FIG. 5C is an optical surge when the optical gain limiter operation is selected.
The conventional optical surge suppressing circuit is constituted as described above. In the former optical surge suppressing method, since gain limiter operation and constant optical output operation are switched based on an input light level, operation at the switching point may become unstable. If a hysteresis is set at the switching point or a large time constant is set to stabilize operation, an optical surge cannot be satisfactorily suppressed for a short signal hit time.
In the latter optical surge suppressing method, although an increase in gain under constant optical output control can be suppressed to a set gain by gain limiter operation upon an input hit, an optical surge determined by the set gain of the limiter is generated.
xe2x80x9cA consideration of optical surge in optical amplifier systemsxe2x80x9d, PROCEEDINGS OF THE IEICE SPRING CONFERENCE, B-941, March 1993, reports that an optical surge generated with a short rise time (msec or less) grows through optical amplifier systems. Even when the gain is suppressed to a set value by gain limiter operation, and the amount of generated optical surge is small, the optical surge grows through optical amplifier systems and thus cannot be ignored.
It is an object of the present invention to provide an optical surge suppressing apparatus for compensating for a level drop of signal light.
It is another object of the present invention to provide an optical surge suppressing apparatus for reliably suppressing generation of an optical surge and improving the reliability of an optical transmission system.
To achieve the above objects, according to the present invention, there is provided an optical surge suppressing apparatus comprising level drop compensating means for compensating for a level drop generated in input signal light, optical surge compressing means for suppressing an optical surge generated in the input signal light, and signal light output means for outputting the input signal light in which the level drop is compensated for by the level drop compensating means and the optical surge is suppressed by the optical surge compressing means.